1. Field of the Invention
The present invention relates to ball screw nut, in particular, to a ball screw nut whose inner part is made of metallic material, while outer part is covered with plastic material.
2. Description of the Prior Art
The ball screw is a mechanical device which has been in use for a long time. A conventional ball screw is essentially composed of a screw bolt, a ball screw nut, and a plurality of rolling balls. The outer surface of the screw bolt is formed into a plurality of spiral grooves, whereas the ball screw nut has a hollow nut body and other structural part for the balls to circulate along. There is a plurality of spiral grooves formed in the inner surface of the nut body coupled with the spiral grooves of the screw bolt to form a pathway for those balls, thus the balls circulate along this pathway formed of the two parts of spiral grooves thereby the relative motion between the screw bolt and nut is carried out with reduced friction. It is well understood that a conventional screw bolt and nut unit is wholly made of metallic material which requires an intricate production process and heat treatment, in addition, fabrication of the associated accessories for a screw bolt and nut unit is also somewhat tricky. As a result, using a screw bolt and nut unit as transmission means is considered to be an expensive measure therefore discouraging wide application in industry.
Aiming at the above depicted problems, the essential object of the present invention is to simplify fabrication process of the screw bolt and nut unit so as to minimize the production cost of the same thereby widening the application field of the screw bolt and nut in industry.
After having carried out a long time research and experimentation, the present inventor came to a conclusion that using metallic material by means of ordinary mechanical process in fabricating the entire ball screw nut will be resulted in a high production cost.
However, there is a problem that the contact between rolling balls and the spiral grooves formed in the screw bolt and nut unit is nearly a point contact made by a spheric steel ball and an inner curved surface of the spiral groove pathway where almost all portion of the loading force of the unit is exerted. In this situation, a metallic material or another equivalently hard material should be employed to fabricate the inner portion of the nut of the screw bolt and nut without compromising.
In the meanwhile, the outer portion, i.e. housing of the ball screw nut is released from a heavy loading force. Accordingly, in the present invention, this part of the ball screw nut is constructed of a plastic material by ejection mold process so as to simplify the fabrication process and lower the production cost.
To enable a further understanding of the innovative and technological content of the invention herein, refer to the detailed description of the invention and the accompanying brief description of the drawings appended below.